1. Field of the Invention
The present invention relates to a dual-circuit hydraulic anti-lock braking system for automotive vehicles, adapted to perform improved anti-lock braking pressure regulation when one of a pair of brake circuits of the dual-circuit hydraulic braking device of the system is functionally lost.
2. Discussion of the Prior Art
A dual-circuit hydraulic braking device is widely used in a motor vehicle which has a pair of groups of wheels. This type of braking device includes a pair of hydraulic brake circuits which are hydraulically independent of each other. One brake circuit is adapted to establish a braking pressure to be applied to a brake cylinder provided for each wheel of a corresponding one of the two wheel groups of the vehicle, while the other brake circuit is adapted to establish another independent braking pressure to be applied to a brake cylinder provided for each wheel of the other wheel group.
It has been known in the art of hydraulic braking devices that, when the braking pressure applied to the wheel brake cylinder for applying brake to the vehicle becomes excessively high in relation to the coefficient of friction of the road surface on which the vehicle is running, a slip amount or rate of the wheel becomes excessively high, which results in an increased braking distance and/or a lowered vehicle running stability. Laid-open Publication No. 58-26661 (published on Feb. 17, 1983) of Japanese Patent Application discloses an anti-lock braking device which is adapted to automatically regulate braking pressure so as to prevent the above-indicated problem.
The above-identified anti-lock braking system includes (a) wheel rotating condition detecting means; (b) vehicle running condition detecting means; and (c) a braking pressure regulating device. The wheel rotating condition detecting means is adapted to detect a rotating condition of at least one wheel of each of the two wheel groups, for example, a rotating speed of the wheel and/or a deceleration rate of the rotating speed of the same, while the vehicle running condition detecting means detects a running condition of the vehicle, for example, a running speed of the vehicle and/or a deceleration rate of the running speed of the same. For example, the vehicle running condition detecting means is constituted by means for presuming the vehicle running speed based on the highest one of the wheel rotating speeds detected by wheel rotating speed detecting means, one of the wheel rotating condition detecting means, and means for calculating the deceleration rate thereof based on the vehicle running speed. Alternatively, the vehicle running condition detecting means is constituted by means for detecting the vehicle running speed and/or the deceleration rate thereof based on detected timewise variation of the roughness or optical image of the road surface on which the vehicle is running. Further, it is disclosed by the above-indicated patent document that the deceleration rate of the vehicle can be detected by an accelerometer fixed to the body of the vehicle. The disclosed braking pressure regulating device, which is adapted to increase and decrease the brake fluid pressure applied to the wheel brake cylinder, initiates the pressure-increasing or pressure-decreasing operation at the moment when the detected wheel rotating speed, deceleration rate or other wheel rotating condition is brought into a predetermined relatioship with the detected vehicle running speed, vehicle deceleration rate or other vehicle running condition, and terminates it when the detected wheel rotating condition is brought into another predetermined condition or state. Usually, at least one of the pressure-increasing and pressure-decreasing operations conducted by the braking pressure regulating device is specified such that it is conducted for a predetermined duration of time from the initiation thereof, that is, it is terminated with passage of the predetermined duration of time. For example, the pressure-decreasing operation is initiated by the regulating device when the actual rotating speed of one of the wheels of the vehicle becomes lower than a target wheel speed which is determined by subtracting a predetermined value indicative of a suitable amount of slip, from the actual vehicle running speed (or which is determined so as to permit the wheel to exhibit a suitable rate of slip), and terminated with passage of a predetermined duration of time. The braking pressure regulating device is capable of performing the pressure-increasing operation in a manner similar to the manner in which to perform the pressure-decreasing operation described above, and further capable of performing both the pressure-increasing and pressure-decreasing operations in that manner.
It is also known that conventional dual-circuit hydraulic anti-lock braking devices suffer from a problem that, in the event of a failure of one of the two hydraulic brake circuits of the device, i.e., when one of the two independent braking pressures is no longer applied to the wheel brake cylinder or cylinders by a corresponding one of the two brake circuits due to its breakage, for example, the other braking pressure transmitted by the other, normally functioning brake circuit, becomes difficult to be appropriately regulated.
Where the pressure-increasing or pressure-decreasing operation is conducted by the braking pressure regulating device for regulating braking pressure for a predetermined duration of time, an amount of increase or decrease of the braking pressure varies depending upon a pressure level of the braking pressure at that time. Because a total braking force of the vehicle is reduced in the event of such a one-circuit failure, a pressure level of the braking pressure transmitted by the normally functioning brake circuit, at the moment when the rotating condition of one wheel of the corresponding wheel group is brought into a predetermined relationship with the vehicle running condition, is different from that of the same at a comparable moment in the case where both the two circuits are normal. For example, in the event of the one-circuit failure a smaller amount of movement of load is produced in a vehicle as a result of applying brake to the vehicle, as compared with when the two brake circuits are normal. Consequently, a smaller and a larger load are exerted to the front and rear wheels of the vehicle, respectively, as compared with when the two circuits are normal, whereby the brake cylinders for the front wheels are subject to a lower braking pressure and the brake cylinders for the rear wheels are subject to a higher braking pressure, at the moment when each wheel is brought into skidding condition.
The Assignee of the present application, in Japanese Patent Application No. 61-222749 (and corresponding U.S. patent application Ser. No. 099,239), has proposed to alter the pressure-reducing time during which the braking pressure is reduced by the braking pressure regulating device, according to the level of the actual deceleration rate of the vehicle (that is, level of the braking pressure) at the time of initiation of the pressure-reducing operation. However, the deceleration rate of the vehicle is varied when one of the two hydraulic brake circuits fails or is functionally lost, more specifically, the vehicle deceleration rate is reduced in the event of the one-circuit failure, as compared with when both the two circuit is normal.
The conventional dual-circuit hydraulic anti-lock braking devices are adapted such that the common pressure-regulating time during which the braking pressure is increased or decreased is used between the case where the two brake circuits are normal and the case where one of the two circuits is functionally lost, whereby the braking pressure is not suitably regulated in the event of the one-circuit failure. Thus, the anti-lock braking pressure regulation conducted by the conventional devices has been unsatisfactory.